The present invention relates to novel coating compositions comprising polyisocyanates, aliphatic oligocarbonate polyols, oligoester polyols and polyacrylate polyols, to a process for their production and to their use in the production of coatings.
Scratch-resistant finishing lacquers, especially for the automotive finishing lacquer sector and for automotive repair lacquering, have already been of great interest for many years. As well as having the property of a low tendency to scratching, for example in a car wash, such finishing lacquer systems are further required to have excellent resistance to solvents and acids.
Accordingly, the market has opened for 2-component (“2K”) polyurethane (“PUR”) systems that are distinguished by good resistance to solvents and chemicals while at the same time having good scratch resistance and excellent weather resistance.
Polyacrylates, optionally in admixture with polyesters as polyol binders, are often used in such systems. Aliphatic and/or cycloaliphatic polyisocyanates based on hexamethylene diisocyanate and/or isophorone diisocyanate are mainly used as crosslinkers.
Such 2K polyurethane coating compositions have achieved a very good overall property level, but scratching of the clear lacquer is frequently observed, especially in the case of dark colors, after frequent washing cycles in car washes. Depending on the elasticity of the lacquer layer, the scratches recede over time, which is referred to as the so-called reflow. However, if the elasticity of the clear lacquer layer is increased in order to improve the reflow behavior, then the lacquer loses surface hardness, and its resistance to solvents and chemicals especially, in particular its resistance to acids, is impaired [Carl Hanser Verlag, Munich, Mo. Metalloberfläche 54 (2000) 60-64]. Attempts are therefore being made in the art to improve the scratch resistance of 2K PUR lacquers by increasing the elasticity of the polyol component, mainly by combinations of polyacrylates and more elastic polyesters.
DE-A 198 24 118 describes low-solvent binders based on polyester-polyacrylate, which can be cured with di- and/or poly-isocyanates to give rapidly drying coatings with good adhesion. However, because of the high polyester content, they have inadequate resistance to acids and are not suitable for use in automotive finishing lacquers.
WO 96/20968 describes a coating composition for cars and heavy goods vehicles which comprises a polyacrylate based on alkyl-substituted cycloaliphatic (meth)acrylate monomers or alkyl-substituted aromatic vinyl monomers, a multi-hydroxy-functional oligoester and a polyisocyanate. However, because the oligoesters, as a result of their preparation, contain a relatively large number of secondary hydroxyl groups in addition to primary hydroxyl groups, and because very large amounts of these esters (>60 wt. %, based on the total formulation) must be used for low-viscosity coating compositions (<3,000 mPa·s/23° C.), they cure fully only very slowly and at relatively high temperatures, so that they are unsuitable for heat-sensitive substrates such as, for example, add-on elements made of plastics.
EP-A 0 896 991 describes coating compositions based on polyacrylate-polyester mixtures having polyester contents ≦10 wt. % and hydroxyl numbers from 40 to 125 mg KOH/g. Owing to the resulting low crosslinking densities, PUR lacquers produced therefrom do not have adequate resistance to solvents and chemicals. Moreover, the viscosity, at 3,000 to 5,000 mPa·s (23° C.) at a solids content of 70 wt. %, is too high for the formulation of high-solids PUR lacquers.
In several documents, such as, for example, in EP 1 101 780 A, EP 819 710 A and EP 778 298 A, mention is often made generally of the use of mixtures of polyacrylates with other polyols, such as, for example, polyesters and/or polycarbonates as polyol binders and reactants for polyisocyanate crosslinkers in 2K PUR lacquers, without giving details of the specific advantages of such mixtures. Furthermore, no information is given about the quantitative composition or the molecular weight and OH functionality of the polycarbonate polyol of such mixed systems.
On page 584 of the anonymous publication 493099 of the Research Disclosure of May 2005, polycarbonate diols and possible combinations thereof with other polyols, as well as corresponding polyurethane coatings, are described. The achievable properties of such lacquers, such as, for example, good adhesion, high gloss, hardness development, flow, resistance to alkalis, flexibility, elasticity, impact resistance and wear resistance, are mentioned generally, without corresponding test results or evidence. However, no information is to be found regarding an improvement in the scratch resistance of corresponding lacquers.
The object of the present invention was, therefore, to provide novel coating compositions which exhibit a marked improvement in scratch resistance without thereby impairing the resistance of the finishing lacquer systems to acids and solvents.
It has been found, surprisingly, that by using specific combinations of aliphatic oligocarbonate polyols, oligoester polyols, specific polyacrylate polyols, and polyisocyanates acting as crosslinkers or curing agents, it is possible to produce coating compositions that exhibit markedly improved scratch resistance while having equally good or even improved resistance to solvents and acids.